The present invention relates to an optical amplifying apparatus constituted by combining Raman amplifiers for amplifying signal light, and an optical communication system utilizing such an optical amplifying apparatus.
In a typical optical communication system as shown in the lower half of FIG. 9, an optical sender (OS) and an optical receiver (OR) are connected by a transmission path, and a plurality of optical repeaters are arranged in the transmission path at required intervals so that signal light repeatedly transmit from the optical sender to the optical receiver. Each of the optical repeaters of this optical communication system is provided with an optical amplifying apparatus including, for example, an erbium-doped optical fiber amplifier (EDFA) and the like so that wavelength-division multiplexed (WDM) signal light including a plurality of optical signals having wavelengths different from one another is collectively amplified.
In the optical amplifying apparatus adopting the aforementioned EDFA, Amplified Spontaneous Emission (ASE) light is generated as the input signal light is amplified, and this ASE light is added to the amplified signal light to be output from the optical amplifying apparatus. In the aforementioned optical amplifying apparatus, total output light power is controlled to be constant, in case of conducting an automatic level control (ALC) such that output light power per single channel wavelength has a predetermined value. As a result, in the optical amplifying apparatus adopting the EDFA, an ASE light component acts as an error component relative to a signal light component to be controlled to a constant level. This error component reduces light power per single channel wavelength of the signal light to be output from the optical amplifying apparatus.
Therefore, with the aforementioned optical communication system, as shown in the upper half of FIG. 9, there has been a problem in that the input signal light power into each succeeding optical amplifying apparatus (a k-th stage in the figure) is reduced whenever the WDM signal light is repeatedly amplified by each preceding optical amplifying apparatus, resulting in deterioration of an optical SN ratio at the optical receiver side. Generally, since an optical amplifying apparatus to be used in a WDM optical communication system has a broadband characteristic, if the number of channel wavelengths of the WDM signal light is used as single channel wavelength, a reduction amount of signal light power due to the ASE light becomes large. To cope with such a reduction of the signal light power when the number of signal light wavelengths is smaller, it may be conceivable to design the system, for example, by previously estimating the deterioration of the signal/noise ratio (SNR) at a smaller number of channel wavelengths of the signal light. However, this reduces a system gain, to thereby problematically reduce a transmittable distance of the system.
To prevent the aforementioned problem, in conventional optical communication systems, a so-called ASE correction (see the upper half of FIG. 9) has been conducted in a manner to calculate the ASE light power by utilizing those information concerning such as a noise figure (NF), input light power, a bandwidth and the number of channel wavelengths of the signal light of an optical amplifying apparatus, so as to increase an output light setting level by an amount corresponding to the calculated ASE light power.
Meanwhile, there has been recently promoted development of an optical amplifying apparatus aiming at spreading an optical amplifying band and at reducing a repeat-loss, by combining an EDFA with a Raman amplifier. In such an optical amplifying apparatus utilizing Raman amplification, noise light due to Raman amplification is generated in addition to the aforementioned ASE light, and this noise light is added to the amplified signal light to be output.
The aforementioned noise light due to Raman amplification is generally called xe2x80x9cRaman scattering light due to pumping lightxe2x80x9d, since such noise light is generated even when only Raman excitation light is input into an amplifying medium in a state where no signal light is input into the amplifying medium. Here, the noise light to be generated in the Raman amplifier shall be called xe2x80x9cAmplified Spontaneous Raman Scattering (ASS) lightxe2x80x9d, in contrast to the Amplified Spontaneous Emission (ASE) light to be generated in an EDFA.
Generation and accumulation of such ASS light causes the same problem as the aforementioned ASE light, in an optical communication system. Thus, it is difficult to prevent deterioration of an optical SN ratio at an optical receiver side, by simply applying the aforementioned conventional ASE correction to an optical amplifying apparatus adopting a combination of an EDFA with a Raman amplifier, thereby requiring a correction also taking account of an influence of ASS light.
The present invention has been carried out in view of the conventional problems as described above, and it is therefore an object of the present invention to provide an optical amplifying apparatus combined with a Raman amplifier for preventing deterioration of a signal/noise ratio of output light, to thereby improve a transmission characteristic of signal light, and an optical communication system utilizing such an apparatus.
To achieve the above object, a first aspect of the optical amplifying apparatus according to the present invention comprises: first optical amplifying means for supplying excitation light to a Raman amplifying medium to thereby Raman amplify signal light propagated through the Raman amplifying medium; and second optical amplifying means for amplifying the signal light output from the first optical amplifying means, wherein the optical amplifying apparatus further comprises controlling means for controlling an operating state of at least one of the first optical amplifying means and the second optical amplifying means, so that a signal/noise ratio of the signal light to be output from the second optical amplifying means is kept substantially constant. Concretely, the signal/noise ratio of the signal light to be output from the second optical amplifying means may be controlled to be substantially constant, irrespective of a noise amount caused in the first optical amplifying means, or irrespective of the number of channel wavelengths of the signal light.
According to such a constitution, one or both of the operating states of the Raman amplification in the first optical amplifying means and the optical amplification in the second optical amplifying means is/are adjusted by the controlling means, so that the signal/noise ratio of the signal light to be output from the second optical amplifying means is controlled to be substantially constant. Thus, it becomes possible to prevent deterioration of the signal/noise ratio due to an influence of noise light generated in the first and second optical amplifying means.
Further, a second aspect of the optical amplifying apparatus according to the present invention comprises: first optical amplifying means for supplying excitation light to a Raman amplifying medium to thereby Raman amplify signal light propagated through the Raman amplifying medium; and second optical amplifying means for amplifying the signal light output from the first optical amplifying means, in which output light of at least one of the first optical amplifying means and the second optical amplifying means is controlled to be a previously determined output setting level, wherein the optical amplifying apparatus further comprises: detecting means for detecting excitation light power to be supplied to the Raman amplifying medium; calculating means for calculating a noise light component due to the first optical amplifying means, based on a detection result by the detecting means; and output setting level correcting means for correcting the output setting level based on a calculation result by the calculating means, to keep signal light power per single channel wavelength included in the output light to be constant, irrespective of the number of channel wavelengths of the signal light.
According to such a constitution, the noise light component due to the first optical amplifying means, i.e., the power of the Amplified Spontaneous Raman Scattering (ASS) light, is calculated by the calculating means based on the excitation light power for Raman amplification detected by the detecting means. Then, the previously determined output setting level of one or both of the first and second optical amplifying means is corrected by the output setting level correcting means based on the calculated ASS light power. Thus, the signal light power per single channel wavelength included in the output light controlled in accordance with the corrected output setting level is kept constant irrespective of the number of channel wavelengths of the signal light, so that the signal/noise ratio of the signal light can be controlled to be substantially constant.
A third aspect of the optical amplifying apparatus according to the present invention comprises: first optical amplifying means for supplying excitation light to a Raman amplifying medium to thereby Raman amplify signal light propagated through the Raman amplifying medium; and second optical amplifying means for amplifying the signal light output from the first optical amplifying means, wherein the optical amplifying apparatus further comprises: detecting means for detecting excitation light power to be supplied to the Raman amplifying medium; calculating means for calculating a noise light component due to the first optical amplifying means, based on a detection result by the detecting means; and transfer means for transferring a calculation result by the calculating means to the second optical amplifying means.
According to such a constitution, the noise light component due to the first optical amplifying means, i.e., the power of the Amplified Spontaneous Raman Scattering (ASS) light, is calculated by the calculating means, based on the excitation light power for Raman amplification detected by the detecting means, and the calculation result is transferred to the second optical amplifying means by the transfer means. Thus, in the second optical amplifying means, it becomes possible to conduct an output correction taking account of an influence of the ASS light.
As a specific configuration of the optical amplifying apparatus according to the first aspect, the controlling means may comprise: an excitation light power detecting section for detecting the excitation light power to be supplied to the Raman amplifying medium; an input light power detecting section for detecting input light power into the second optical amplifying means; control information receiving section for receiving information concerning a signal/noise ratio at a preceding stage optical amplifying apparatus; and a calculation controlling section for obtaining the signal/noise ratio of the signal light to be output from the second optical amplifying means, based on the noise light component due to the first optical amplifying means calculated corresponding to the detection result by the excitation light power detecting section and to the information received by the control information receiving section, and based on a noise light component due to the second optical amplifying means calculated corresponding to the detection result by the input light power detecting section and to the information received by the control information receiving section, and for controlling the operating state of at least one of the first optical amplifying means and the second optical amplifying means, corresponding to the thus obtained signal/noise ratio.
According to such a constitution, a caused amount of the noise light component due to the Raman amplification is calculated by the calculation controlling section, corresponding to the Raman excitation light power detected by the excitation light power detecting section and the information concerning the signal/noise ratio (preferably, the information including the signal/noise ratio and the number of multiplexed channel wavelengths for the signal light) at the preceding stage optical amplifying apparatus, received by the control information receiving section. Further, a caused amount of the noise light component due to the amplification by the second optical amplifying means is calculated by the calculation controlling section, corresponding to the input light power into the second optical amplifying means detected by the input light power detecting section and the information received by the control information receiving section. Then, the signal/noise ratio of the signal light to be output from the second optical amplifying means is obtained, based on the calculated respective noise light components, the operating state of at least one of the first and second optical amplifying means is adjusted, corresponding to the thus obtained signal/noise ratio, and the signal/noise ratio of the signal light to be output from the second optical amplifying means is controlled to be substantially constant.
Further, when the first optical amplifying means or the second optical amplifying means includes a constant-output controlling section for controlling total power of the output light to be constant, the controlling means of the optical amplifying apparatus may control the output setting level of the constant-output controlling section. In addition, the controlling means may control setting of an amplification gain in the second optical amplifying means.
Moreover, the second optical amplifying means of the optical amplifying apparatus may comprise an optical fiber amplifier adopting a rare earth element doped optical fiber. As a specific configuration in this case, it is possible to connect in series a plurality of optical amplifying sections having substantially the same amplifying wavelength band, or to connect in parallel a plurality of optical amplifying sections having different amplifying wavelength bands. In the above constitution, the caused amount of the Amplified Spontaneous Emission light (ASE light) due to the optical fiber amplifier adopting the rare earth element doped optical fiber is to be calculated by the controlling means.
The optical communication system according to the present invention is constituted to include a plurality of optical repeaters, each having the aforementioned optical amplifying apparatus, arranged in a transmission path connecting an optical sender and an optical receiver. According to such a constitution, the signal light having the signal/noise ratio controlled to be substantially constant is output from each optical repeater and repeatedly transmitted, to thereby provide excellent receiving sensitivity at the optical receiver.